(P128) Statistically Significant Calibration Curve for Ir-192 HDR Source Using Multichannel Gafchromic Film Analysis

J. Barbiere, J. Napoli, A. Ndlovu; HUMC

PURPOSE: This work presents a unique methodology to create statistically valid calibration curves required to validate high-dose-rate (HDR)–planned dose distributions during brachytherapy treatment planning commissioning. A robust calibration curve is essential to accurately convert measured planar Gafchromic film density to dose for comparison with calculated dose distributions using standard metrics. The process presented herein is appropriate for widespread application since it can be used with any planning/analysis system that is capable of DICOM (Digital Imaging and COmmunication in Medicine) dose import/export, does not require specialized phantoms, and can be performed rapidly on the actual film sheet used for plan validation in a single exposure.

MATERIALS AND METHODS: A section of Gafchromic EBT3 film was positioned in contact with an HDR bronchial catheter surrounded by water-equivalent material. The film was irradiated for 1,350 Cisec with a Nucletron Ir-192 HDR line source and then digitized with an Epson 10000 XL color scanner. The remaining section of the film can be used for plan evaluation.

Film analysis was performed in MatLab with the imaging toolbox. Transmission values for the red, green, and blue channels were converted into optical density (OD) as a function of perpendicular distance from the center of the line source. A corresponding theoretical dose curve for each exposure was calculated using the TG43 formalism. The calculated dose was fitted to the measured OD in the clinically significant range to a fourth-order polynomial.

A single dwell position plan, normalized for the appropriate irradiation time, was created using the Oncentra treatment planning system. The three-dimensional (3D) DICOM dose matrix was imported into a Varian Eclipse external beam treatment planning system, and the two-dimensional (2D) dose in a coronal plane was extracted for comparison in MatLab with the corresponding measured film dose plane.

RESULTS: (1) The green channel had greater range, contrast scale, and accuracy than either the blue or red channel and was therefore used for analysis. (2) The calibration curve, consisting of nearly 200 points, was statistically valid and robust. (3) Agreement between the measured film and plan dose was within +/− 1.5% throughout the clinical range in 0.1-mm increments. (4) Our results provide a significant improvement in both dose and distance to agreement over current practice, which typically uses fewer than 10 calibration points.

CONCLUSION: An accurate and robust calibration curve using the green channel of Gafchromic film can be easily created for use in validating HDR planning systems.

Proceedings of the 97th Annual Meeting of the American Radium Society - americanradiumsociety.org